Beta-lactam antibiotics are characterized by a beta-lactam ring in their molecular structure. The integrity of the beta-lactam ring is essential for the biological activity, which results in the inactivation of a set of transpeptidases that catalyze the final cross-linking reactions of peptidoglycan synthesis. Members of the beta-lactam antibiotics family comprise penicillins, cephalosporins, clavams (or oxapenams), cephamycins and carbapenems.
Beta-lactamases are bacterial defensive enzymes that hydrolyze beta-lactam antibiotics. The production of beta-lactamases is a predominant mechanism to confer beta-lactam resistance in Gram-negative bacteria. Beta-lactamases catalyse very efficiently irreversible hydrolysis of the amide bond of the beta-lactam ring resulting in biologically inactive product(s).
Because of the diversity of enzymatic characteristics of different beta-lactamase types, several classification systems have been proposed for their categorizing. The classifications are based on two major approaches, which are functional and molecular classifications.
The functional classification scheme of beta-lactamases proposed by Bush et al., (1995, Antimicrob. Agents Chemother. 39: 1211-1233) defines four beta-lactamase groups, which are based on their substrate and inhibitor profiles. Group 1 consists of cephalosporinases that are not well inhibited by clavulanic acid. Group 2 consists of penicillinases, cephalosporinases and broad-spectrum beta-lactamases that are generally inhibited by active site-directed beta-lactamase inhibitors. Group 3 consists of metallo-beta-lactamases that hydrolyze penicillins, cephalosporins and carbapenems, and that are poorly inhibited by almost all beta-lactam-containing molecules. Group 4 consists of penicillinases that are not well inhibited by clavulanic acid. Subgroups have also been defined according to rates of hydrolysis of carbenicillin or cloxacillin (oxacillin) by group 2 penicillinases.
The most widely used classification is Ambler classification which divides beta-lactamases into four classes (A, B, C, D) and is based on their amino-acid sequences (Ambler 1980, Philos Trans R Soc Lond B Biol Sci. 289: 321-331). Classes A, C, and D gather evolutionarily distinct groups of serine beta-lactamase enzymes, and class B the zinc-dependent (“EDTA-inhibited”) beta-lactamase enzymes (Ambler R. P. et al., 1991, Biochem J. 276: 269-270). Classes A, C, and D belong to serine beta-lactamases, in which the hydrolysis of the beta-lactam is mediated by serine in an active site. Serine beta-lactamases are related to DD peptidases (D-alanyl-D-alanine carboxypeptidase), the target enzyme of beta-lactams. The mechanism by which serine beta-lactamases hydrolyze beta-lactam antibiotics is believed to follow a three-step pathway including a non-covalent Henri-Michaelis complex, a covalent acyl-enzyme intermediate and deacylation (Matagne et al., 1998, Biochem J 330:581-598). Acylation mechanism is considered to be a common mechanism for all serine beta-lactamase groups whereas, on the basis of theoretical calculations, the substrate deacylation mechanisms of serine beta-lactamase of classes A, C and D appear to differ from each other. Deacylation mechanisms have both common and group specific elementary processes (Hata M et al., 2006, Biol Pharm Bull. 29: 2151-2159).
Bacillus spp. serine beta-lactamases and TEM-1, SHV-1 and CTX-M families have primarily been classified as class A beta-lactamases and as penicillinases that possess good capability to hydrolyze e.g. penicillin and ampicillin. The class A beta-lactamases were first identified in penicillin resistant St. aureus in the 1940s. A plasmid-borne penicillin resistance gene, TEM-1, was discovered in E. coli 20 years later. Later on, serine beta-lactamases were also shown to evolve the ability to hydrolyze most cephalosporins and further specialize at hydrolysing a specific subset of cephalosporins. Most of these extended-spectrum beta-lactamases (ESBL) are derivates of TEM-1, TEM-2 or SHV-1 enzymes. Recently there are increasing numbers of reports that describe the vast emergence of CTX-M enzymes, a new group of class A ESBLs. Nowadays, CTX-M enzymes are the most frequently observed ESBLs and are sub-classified into five major families. CTX-M enzymes have a wide substrate range including penicillin and the first, second and third generation cephalosporins (Bonnet, R. 2004. Antimicrob Agents Chemother. 48:1-14).
While the sequence similarity between the class A beta-lactamases (TEM, SHV, CTX-M, Bacillus spp. beta-lactamases) is moderate, the crystal structures of all serine beta-lactamases show a particularly high similarity (Matagne et al., 1998, Biochem J 330:581-598; Tranier S. et al., 2000, J Biol Chem, 275: 28075-28082; Santillana E. et al., 2007, Proc Natl Acad Sci. USA, 104: 5354-5359). The enzymes are composed of two domains. One domain consists of a five-stranded beta sheet packed against three alpha helices whilst the second domain, an alpha domain, is composed of eight alpha helices. The active site pocket is part of the interface between these two domains and is limited by the omega loop. The omega loop is a conserved structural element of all class A beta-lactamases and is essentially involved in catalytic reaction (FIG. 1).
Several conserved peptide sequences (elements) related to catalysis or recognition of the substrate have been identified in class A beta-lactamases. The first conserved element 70-Ser-X-X-Lys-73 (SEQ ID NO: 17) (Ambler classification) includes the active serine residue at location 70 in alpha helix2 and lysine at position 73. The second conserved element is a SXN loop in an alpha domain (at positions between 130 and 132 according to Ambler classification), where it forms one side of a catalytic cavity. The third conserved element (at positions between 234 and 236 according to Ambler classification) is on the innermost strand of the beta-sheet3 and forms the other side of the catalytic cavity. The third conserved element is usually KTG. However, in some exceptional cases, lysine (K) can be replaced by histidine (H) or arginine (R), and in several beta-lactamases, threonine (T) can be substituted by serine (S) (Matagne et al., 1998. Biochem J 330:581-598).
Beta-lactamase mediated resistance to beta-lactams is widely spread among pathogen and commensal microbiota, because of abundant use of beta-lactams in recent decades. Indeed, antibiotic resistance is a well-known clinical problem in human and veterinary medicine, and hundreds of different beta-lactamases derived from Gram-positive and Gram-negative bacteria have been purified and characterized in the scientific literature. Because the use of antimicrobials has not reduced and furthermore, antimicrobial resistance has become part of the everyday life, new approaches are invariably and urgently required for solving these medical problems.
The intestinal microbiota of humans is a complex bacterial community that plays an important role in human health, for example, by stimulating the immune response system, aiding in digestion of food and preventing the overgrowth of potential pathogen bacteria. Antimicrobial agents e.g. beta-lactams are known to have effect on normal microbiota. The efficacy of antimicrobial agents to promote changes of normal intestinal microbiota is associated with several factors including drug dosage, route of administration and pharmacokinetics/dynamics and properties of antibiotics (Sullivan Â. et al., 2001, Lancet 1:101-114). Even though the intestinal microbiota have a tendency to revert to normal after completion of antibiotic treatment, long term persistence of selected resistant commensal bacteria has been reported (Sjölund M. et al., 2003, Ann Intern Med. 139:483-487). Such persistence and the exchange of antibiotic resistance genes make the commensal microbiota a putative reservoir of antibiotic resistance genes.
Certain parentally administered beta-lactams like ampicillin, ceftriaxone, cefoperazone, and piperacillin are in part eliminated via biliary excretion into the proximal part of the small intestine (duodenum). Residual unabsorbed beta-lactams in the intestinal tract may cause an undesirable effect on the ecological balance of normal intestinal microbiota resulting in antibiotic-associated diarrhea, overgrowth of pathogenic bacteria such as vancomycin resistant enterococci (VRE), extended-beta-lactamase producing Gram-negative bacilli (ESBL), Clostridium difficile, and fungi, and selection of antibiotic-resistance strains among both normal intestinal microbiota and potential pathogen bacteria.
The therapeutic purpose of beta-lactamases is inactivating unabsorbed antibiotics in the gastrointestinal tract (GIT), thereby maintaining a normal intestinal microbiota and preventing its overgrowth with potentially pathogenic micro-organisms (WO 93/13795).
There are at least three requirements for beta-lactamase drug products, which are suitable for GIT targeted therapy. The first requirement is to preserve enzymatic activity under conditions prevailing in the GIT. Resistance against proteolytic breakdown by various proteases secreted from various glands into the GIT is a quintessential precondition for the feasibility of beta-lactamase therapy. Another important consideration is the range of pH values prevailing in the different compartments of the small intestine. These pH values usually vary between 5 (duodenum) and 7.5 (ileum). Hence, in order to qualify as candidates for the intended therapeutic purpose, a beta-lactamase needs to exhibit high enzymatic activity over the pH range 5-7.5.
The second requirement of a beta-lactamase or a product thereof is to hydrolyze beta-lactam efficiently. The concentration of a beta-lactam antibiotic in small intestinal chyme during an antibiotic treatment episode is mostly related to the elimination of the particular beta-lactam via biliary excretion. A suitable beta-lactamase needs to have kinetic parameters that enable it to effectively hydrolyze lower GIT beta-lactam concentrations below levels causing alterations in intestinal microbiota. The ideal set of kinetic parameters consists of a numerical low value for the Michaelis constant KM, combined with a numerically high value for the maximum reaction rate Vmax. A high Vmax value is required in order to provide a sufficient degree of breakdown capacity, while a low KM value is needed to ensure beta-lactam degrading activity at low substrate concentrations.
The third requirement of a beta-lactamase or a product thereof is to tolerate the conditions, such as relatively high temperatures, in the manufacturing of pharmaceutical compositions. Moreover, in the production process, the mixing dispersion of aqueous excipients and drug substance requires a high degree of solubility at suitable pH.
An enzymatic therapy, named Ipsat P1A, is being developed for the prevention of the adverse effects of β-lactam antibiotics inside the gut. Ipsat P1A delivery system has been designed to inactivate parenterally given penicillin group beta-lactams (e.g. penicillin, amoxicillin ampicillin and piperacillin) with or without beta-lactamase inhibitors (e.g. tazobactam, sulbactam, clavulanic acid) excreted via biliary system (WO 2008065247; Tarkkanen, A. M. et al., 2009, Antimicrob Agents Chemother. 53:2455-2462). The P1A enzyme is a recombinant form of Bacillus licheniformis 749/C small exo beta-lactamase (WO 2008065247) which belongs to class A and is grouped to subgroup 2a in functional classification. B. licheniformis beta-lactamase and its P1A derivate are considered as penicillinases which have high hydrolytic capacity to degrade e.g. penicillin, ampicillin, amoxicillin or piperacillin (Table 1) and they are generally inhibited by active site-directed beta-lactamase inhibitors such as clavulanic acid, sulbactam or tazobactam (Bush K. et al., 1995, Antimicrob Agents Chemother 39: 1211-1233).
However, the P1A enzyme has only a very limited capacity to inactivate beta-lactam antibiotics that belong to the cephalosporin or the carbapenem group. Because the employed beta-lactamases possess poor activity to cephalosporins, they can not be applied in conjunction with parenteral cephalosporin therapy for inactivation of unabsorbed beta-lactam in the small intestinal tract.
Therefore, new beta-lactamases or derivates of P1A with extended substrate profile, for example as seen in metallo-beta-lactamases, are indispensable.
The present invention provides novel genetically tailored derivates of P1A beta-lactamase and furthermore, novel methods for modifying and producing beta-lactamases.